The present invention relates to stretchable wrapping machines for wrapping various size loads with a stretchable wrapping material.
Exemplary of the type of stretchable wrappers in conjunction with which the present invention can be utilized are those types of machines such as broadly disclosed in U.S. Pat. Nos. 3,867,806 and 4,050,221 to Lancaster et al., the subject matters of which are hereby incorporated by reference. However, the minimum tension mechanism utilized during the initiation of the wrapping operation in accordance with these patents to Lancaster et al. need not be utilized in the operation of the wrapping machine of the present invention.
In many of the stretch wrapping machines currently marketed, variable speed turntable drive mechanisms are included as optional equipment. Such variable speed drive mechanisms are included in order to enable a gradual start in acceleration of the motor up to the set speed upon initiation of the wrapping operation. Typically during initiation of the wrapping operation, the wrapping material is merely tucked into the load and hence is not securely attached to the load. Consequently if the initiation of the wrapping operation involved rapid acceleration to the set operating speed, i.e. a rapid start operation, the wrapping material might be pulled out of the load hence necessitating the entire wrapping operation to be restarted. The utilization of such a soft or slow start feature avoids such problems. During such a soft start the tension on the material is normally set at about at least 50% of the full tension during the wrapping operation.
When stopping such wrapping operations in these machines, however, it has been the common practice to simultaneously deactivate the motor and to activate a braking mechanism for immediately stopping the rotation of the support. The utilization of such a braking mechanism is disclosed in U.S. Pat. No. 3,626,654 to Hoffler et al., the subject matter of which is hereby incorporated by reference.
Such a rapid braking of the rotational movement of the support turntable leads to several potential problems. First, there is a tendency for the turntable to overshoot the desired stopping location. This tendency becomes a particularly significant problem where the stretch wrapper is used as part of a conveyor system. In such conveyor systems, proper alignment of the support turntable with the infeed and discharge conveyors is critical. In order to prevent such an overshooting of the turntable, some manufacturers of stretch wrapping machines, such as Infra-Pak, have employed a positive alignment table locking device. Another problem that can occur from the rapid stopping of the support turntable is the effect upon the stretch material and the withdrawal of the stretch material from the dispensing mechanism. The rapid stopping of the support turntable without insuring that the dispensing of the material from the dispensing mechanism is also ceased can cause excess material to be withdrawn from the dispensing mechanism which generates a slack of material thereby reducing the tension on the material and its ability to tightly secure the load being wrapped. Furthermore, the rapid stopping of the support turntable generates a shock to the machine which increases the amount of maintenance needed for the machine and reduces the life of the machine. Other problems that result from the use of such a rapid stopping mechanism will be apparent from the discussion of the advantages of the present invention as set forth further below.
While variable speed drive mechanisms have been extensively employed, such as in those patents noted above, such drive mechanisms have not been used so as to reduce the speed of the support turntable in a stretchable wrapping machine during a short time period immediately prior to stopping the rotation of the support turntable so as to provide a gradual stopping or soft-stop of the support turntable. Other patents that illustrate variable speed drive mechanisms for varying the speed of operation of rotating a support during a winding or unwinding operation are illustrated in U.S. Pat. Nos. 3,977,621 to Huffman, 4,003,525 to Podvin et al. and 4,103,840 to Dowd. In each of these machines, the variable speed drive that may include the utilization of a braking mechanism are used for insuring that a constant tension is maintained on the material being either wound or unwound from a dispensing mechanism and so as to prevent spillage of the material from the dispensing mechanism. None of these patents, however, provide any disclosure of a stretch wrapping machine that is either concerned with or solves those problems previously discussed above that occur in the termination of the wrapping operation in a stretch wrapping machine as previously discussed above.